Infrared Reflection Adsorption Spectroscopy Research Articles

Structural changes of silicon dioxide films caused by synchrotron irradiation

Effect of synchrotron radiation (SR) on silicon oxide films, deposited with remote plasma-enhanced chemical-vapor deposition, has been investigated. SR irradiation had an effect on the silicon dioxide structure which was confirmed by in situ infrared reflection adsorption spectroscopy. As the substrate temperature was increased to 373 K, the SiO2 structure became loose by irradiation and surface migration was observed. Surface migration of species induced by SR irradiation led to the increase of surface roughness and also the formation of hemispherical bubbles on the film surface, which was confirmed by atomic force microscope. Generation of oxygen-related paramagnetic defect was observed by electron spin resonance.

Adsorption of CO on Pd/Al2O3/Ta(110) model catalysts

The adsorption of CO on Pd/Al2O3/Ta(110) model catalysts was studied using infrared reflection adsorption spectroscopy (IRAS) for two different Pd coverages, corresponding to 25–30 and 60–70 Å average particle sizes. Striking similarities between the IRAS data for the larger particles and Pd single crystals indicate that the particles are comprised predominantly of 〈111〉 and 〈100〉 facets. The isosteric heat of adsorption of CO extrapolated to zero coverage was determined to be 38.9 kcal/mol on the 60–70 Å particles. IRAS and Auger results provide evidence of CO dissociation on the smaller particles.

Novel properties of ultrathin oxide films: NiO supported on Al2O3

Ultrathin NiO films have been prepared on an Al2O3(0001) surface, supported on a Mo(110) or Re(0001) substrate. The electronic and chemical properties of these NiO films have been studied using electron energy loss spectroscopy (ELS), temperature programmed desorption (TPD), and infrared reflection adsorption spectroscopy (IRAS). TPD and IRAS of adsorbed CO show that the chemical properties of the thin ( 10 monolayers) NiO films. The electronic structure, however, for NiO films with thicknesses between 0.5 and 28 ML are indistinguishable by ELS. Heating monolayer NiO on Al2O3 to 800 K in vacuum leads to a substantial reduction of NiO to metallic Ni.

Model Studies in Catalysis Using Surface Science Probes

Despite successes in modeling catalysts with single crystals, there is a clear need to develop models with higher levels of complexity to address those important issues specifically related to very small metal particles and to interactions between the metal particle and the support. This article begins with a discussion of selected examples of single-crystal work and concludes with a description of recent efforts to simulate oxide-supported metal systems. These examples illustrate the wide range of opportunities offered by model systems for studying complex issues of catalysis such as structure sensitivity/insensitivity and metal-support interactions. In the latter case, by using a model oxide-supported metal catalyst, the nuances of the more complex real world system can be directly investigated while maintaining a catalytic system that is tractable for typical surface science probes. The experiments were carried out utilizing several combined ultrahigh vacuum/microreactor systems, described elsewhere, with capabilities for X-ray and UV photoelectron spectroscopy (XPS, UPS), ion scattering spectroscopies (ISS), high-resolution electron energy-loss spectroscopy (HREELS), infrared reflection adsorption spectroscopy (IRAS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD), and facilities for sample heating and cooling. 84 refs.

SEGREGATION EFFECTS IN THE OXYGEN/CYCLOHEXANE/Ni{111} COADSORPTION SYSTEM

The coadsorption of cyclohexane and oxygen on Ni{111} at 110 K was studied using reflectionadsorption infrared spectroscopy (RAIRS). The effect of postdosed oxygen on the cyclohexane adlayer can be understood in terms of repulsive oxygen-cyclohexane interactions which lead to segregation and compression of the cyclohexane islands.

Spectroscopic studies of the interface built up during the formation of an electropolymerized polyacrylonitrile thin film on a copper surface cathode

AbstractWe present in this work a series of result obtained on the atmospherically oxidized copper‐acrylonitrile system, our aim being to follow the deposition, by grafting and growing, of electropolymerized polyacrylonitrile film. Combining analytical means of electrochemistry—XPS, UPS and infrared reflection adsorption spectroscopy (IRAS)—allows quite an accurate description of this phenomena.The film was deposited by the imposition of a linearly growing negative tension to the copper electrode dipped in an electroyltic solution containing acrylonitrile. The polarization is stopped at different potentials of the current‐potential characteristic. These potentials correspond to the formation steps of the interface. Thus we describe, successively, the nature of the initial states (atmospherically oxidized copper) and the final states (insulating and covered polyacrylonitrile thin film of 20–30 Å) in relation to the change in density of occupied electronic superficial states.The appearance of this insulating and covering film is expressed on the current‐potential characteristic by an inhibition peak, named and passivation peak.Moreover, the non‐stoichiometric character of the superficial copper oxide is identified and permits the use of a low electrochemical potential (before the oxide electroreduction potential) to graft the polymer onto non‐oxidized sites.

Carbon-carbon coupling of methyl groups on Pt(111)

Although methyl radicals are widely postulated to be important intermediates in many catalytic processes involving hydrocarbon species, relatively little is known about their reaction kinetics on metal surfaces. In order to more fully understand the surface chemistry of methyl groups we have developed a source of gas-phase methyl radicals based on the pyrolysis of azomethane. The surface chemistry of methyl groups adsorbed on Pt(111) has been studied using TPD and reflection-adsorption infrared spectroscopy (RAIRS). Methyl radicals can be dosed directly onto the surface at 150 K and are bound in a C 3v geometry. Methyl groups become thermally activated above 230 K, reacting to produce a hydrogenation product (methane) and various dehydrogenation products. The nature of these dehydrogenation products depends upon the initial methyl coverage. At low coverages CH x ( x 3) species are predominant while at higher methyl coverages is evidence of carbon-carbon coupling reactions to produce higher order hydrocarbons one of which has been identified as ethylidyne (CCH 3) Ethylidyne is also observed as a reaction product on Pt(111), at 150 K, for large methyl radical exposures.

Infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed desorption study on the absorption and decomposition of Fe(CO)5 over silver surfaces

The adsorption and decomposition of Fe(CO)5 over Ag surfaces have been studied using infrared reflection adsorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The thermal desorption spectrum (TDS) showed that Fe(CO)5 is partly chemisorbed at temperatures below 120 K on the Ag surface cleaned by Ar-ion sputtering. The active sites for chemisorption may be defect sites produced by the sputtering. XPS revealed that the chemisorbed species involve a small amount of dissociated species even at 95 K and undergo decarbonylation upon heating the substrate to ca. 180 K to form an intermediate species, Fe(CO)4, which is stable at temperatures up to 280 K and then decomposed completely above 330 K, depositing Fe metal on the surface. An intense C—O stretching band at 2062 cm–1 and a weak band at 2116 cm–1 in the IRA spectra of adsorbed Fe(CO)5 decrease and eventually disappear upon elevating the temperature to 230 K, and a new band grows at 2052 cm–1 upon heating the substrate from 230 to 280 K. These IRAS results were discussed in terms of a surface selection rule for IRAS. The geometry of adsorbed Fe(CO)5 was concluded to be square pyramidal with the square base toward the surface, and the Fe(CO)4 moiety trigonal pyramidal. The deposited Fe metal from Fe(CO)4 was found to dissolve to some extent into the bulk Ag at temperatures below 400 K.

Structural, chemical and electronic properties of copper/tantalum (110)

The properties of ultrathin Cu films on a Ta(110) substrate have been studied using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and infrared reflection adsorption spectroscopy (IRAS). For coverages of <1.2 monolayer the Cu overlayers grow pseudomorphic with respect to the Ta substrate. XPS results indicate that the Cu(2p[sub 3/2]) binding energy of the supported Cu atoms is perturbed by +0.3 eV compared to that of the surface atoms in Cu(100). These electronic and physical perturbations induce distinct differences in the chemisorptive properties of the Cu films. This is evidenced by CO TPD results which indicate a 50 K increase in the CO desorption temperature as well as a CO stretching frequency that is 20 cm[sup [minus]1] higher than observed for the Cu(111) surface. A comparison of these results with those previously obtained for Cu overlayers on different substrates shows a correlation among the electronic perturbations of the adlayers, the metal-substrate bond strengths, the CO desorption temperatures, and the C-O stretching frequencies. 60 refs., 13 figs.

A reflection-absorption infrared spectroscopy study of the adsorption of dioxygen species on a silver surface

Two, adsorbed molecular-oxygen species are identified on a polycrystalline silver surface using reflection-adsorption infrared spectroscopy (RAIRS). One oxygen species is a peroxo (O 2− 2) species which is identified from its vibrational frequency (983 cm −1) and which stands up on the surface. Its heat of adsorption, measured from adsorption isosteres, varies with oxygen coverage and is given by ΔH (ads)=14.5–18.7 θ kcal/ mol where θ is the coverage and θ=1 is measured from the integrated intensity of the 983 cm −1 RAIRS peak for a sample temperature of 90 K in the presence of 145 Torr of oxygen. The second oxygen species is oriented with its OO bond parallel to the surface, has an OO stretching frequency of 622 cm −1 and is postulated to be the precursor to dissociative oxygen adsorption. Its heat of adsorption is also coverage dependent being given by ΔH (ads)=14.0–16.3 θ kcal/ mol where here θ=1 is taken as the maximum integrated peak intensity for an oxygen pressure of 250 Torr at a sample temperature of 90 K. The variation in coverage for both species as a function of oxygen pressure is well described by a Temkin isotherm in accord with their coverage-dependent heats of adsorption.

Cu titration of tilted CO on a Mo(110) surface

Upon CO exposure to a Mo(110) surface at 95 K, the initial ≈0.3 monolayer (ML) of CO does not exhibit a vibrational frequency in the 1800–2200 cm − region, implying that the CO axis is tilted with respect to the surface normal. Upon subsequent deposition of ≈0.9 ML Cu at 95 K followed by heating, infrared-reflection-adsorption-spectroscopy (IRAS) shows that a portion of the tilted CO molecules diffuses onto the Cu overlayer and assumes an upright bonding geometry, with the remainder dissociating on the Mo(110) surface into atomic C and O.

Ionic transport effects in in situ Fourier-transform infrared reflection adsorption spectroscopy

Ionic transport effects in in situ Fourier-transform infrared reflection adsorption spectroscopy

Infrared reflection-adsorption spectroscopy of W(100)-H at 100 K

Using infrared reflection-absorption spectroscopy in conjunction with the broad-band technique of Fourier-transform interierometry, changes m the absorption spectrum of W(100) have been investigated as a function of H coverage at 100 K from 900 to 3900 cm −1. The changes measured upon initial adsorption indicate the removal of intrinsic-surface states 0.36 eV below the Fermi surface as well as the establishment of the unreconstructed-surface electronic absorption at ∼ 0.15 eV. The appearance of the 0.15 eV mode upon initial adsorption implies that the surface largely switches to an unreconstructed state at low coverages, consistent with H-atom immobility at low temperatures. For higher coverages [> 0.7 monolayer (ML) (2.0 ML  saturation)] the measured reflectivity changes show that free-carrier surface scattering dominates the coverage induced variations. It is seen that infrared measurements of these surface-scattering variations can lead to determination of the effective plasma frequency ohmsp of the metal.

Decomposition of methanol over oxidized and reduced copper surfaces studied by double modulation Fourier transform infrared reflection absorption spectroscopy

Adsorption and decomposition of methanol over oxidized and reduced copper surfaces with or without vapor phase were observed by double modulation Fourier transform infrared reflection adsorption spectroscopy (DM FT-IRAS). Over the oxidized copper surface, methanol is adsorbed to form a formate ion and decomposes to CO 2 and H 2O at 423 K. Over the reduced copper surface, adsorbed species of methanol were physically adsorbed methanol, methoxide, and polyoxymethylene after the reaction at 473 K and measured at 300 K with the vapor phase. The decomposition products of methanol at 473 K are CO, H 2 and methylformate. The decomposition path of methanol over the oxidized and reduced Cu surfaces are discussed.

Adsorption of glycine on hydrophilic gold

The adsorption of glycine (H 3N +CH 2CO − 2) onto evaporated gold substrates, treated to be hydrophilic, has been investigated using infrared reflection adsorption spectroscopy (IRAS) and angle-dependent X-ray photoelectron spectroscopy (XPS). Films only a few monolayers in thickness were formed by adsorption from a 0.1 mg/ml aqueous solution at pH 5.7. These films were characterized by strong infrared absorption bands at 3185 and 1528 cm −1 that were attributed to asymmetric stretching and symmetric deformation modes of the NH + 3 group. The asymmetric stretching mode of the CO − 2 group also appeared as a strong band near 1620 cm −1, while very weak bands were observed in regions where the corresponding symmetric stretching and scissoring modes are expected to absorb. These results indicated that a preferentially oriented film of zwitterionic glycine was formed on gold with the CC and CN bonds oriented essentially parallel and perpendicular to the metal surface, respectively. From the binding energy of the N(1 s) peak in the XPS spectrum, the species on the gold surface can be identified as zwitterionic glycine. The angle-dependent intensity of the N(1 s) relative to O(1 s) and C(1 s) spectra indicated that the zwitterionic species tended to be oriented with the N +-end closest to the gold surface.

Platinum (410), a face with unusual activity for the breaking of N-O and C-O bonds

Banholzer et al., Surface Sci. 128 (1983) 176, suggested that ideas, analogous to those proposed by Woodward and Hoffmann for reactions in organic systems might be useful in identifying especially promising catalysts structures for further study. This work shows that Pt(410) has sites with the right orbital symmetry to break NO and CO bonds. Recently X-ray photoemission spectroscopy (XPS), temperature programed desorption (TPD), auger electron spectroscopy (AES), and reflection adsorption infrared spectroscopy (RAIRS) studies were done to see if these sites do indeed have unusual properties for N-O and C-O bond breaking. XPS showed that NO dissociates upon adsorption on Pt(410) at room temperature. RAIRS indicates that there is however, a small fraction (≈ 20%) of the NO which molecularly adsorbs. TPD indicates that more than 90% of the NO which adsorbs dissociates before it desorbs. By comparison, the next most active face considered previously shows negligible dissociation at room temperature, and only 50% dissociation upon heating. Surprisingly however, nitrogen and oxygen bond energies inferred from flash desorption peak temperatures are lower on Pt(410), than on other less active faces (Pt(100), Pt(110)). CO was found to adsorb molecularly at room temperature. However, heating the layer to 500 K results in changes in the XPS spectra that are consistent with dissociation. No other platinum surface studied previously shows similar behavior. RAIRS indicates that initially the CO adsorbs in a linearly bound state. However, readsorption on a partially dissociated layer results in the formation of bridge bound CO. Clearly, then Pt(410) is unusually active. These results show that simple orbital symmetry arguments can be used to explain the relative activity of single crystal planes, and identify planes with potential for enhanced activity for study.

Platinum (410), a face with unusual activity for the breaking of N-O and C-O bonds

Banholzer et al., Surface Sci. 128 (1983) 176, suggested that ideas, analogous to those proposed by Woodward and Hoffmann for reactions in organic systems might be useful in identifying especially promising catalysts structures for further study. This work shows that Pt(410) has sites with the right orbital symmetry to break NO and CO bonds. Recently X-ray photoemission spectroscopy (XPS), temperature programed desorption (TPD), auger electron spectroscopy (AES), and reflection adsorption infrared spectroscopy (RAIRS) studies were done to see if these sites do indeed have unusual properties for N-O and C-O bond breaking. XPS showed that NO dissociates upon adsorption on Pt(410) at room temperature. RAIRS indicates that there is however, a small fraction (≈ 20%) of the NO which molecularly adsorbs. TPD indicates that more than 90% of the NO which adsorbs dissociates before it desorbs. By comparison, the next most active face considered previously shows negligible dissociation at room temperature, and only 50% dissociation upon heating. Surprisingly however, nitrogen and oxygen bond energies inferred from flash desorption peak temperatures are lower on Pt(410), than on other less active faces (Pt(100), Pt(110)). CO was found to adsorb molecularly at room temperature. However, heating the layer to 500 K results in changes in the XPS spectra that are consistent with dissociation. No other platinum surface studied previously shows similar behavior. RAIRS indicates that initially the CO adsorbs in a linearly bound state. However, readsorption on a partially dissociated layer results in the formation of bridge bound CO. Clearly, then Pt(410) is unusually active. These results show that simple orbital symmetry arguments can be used to explain the relative activity of single crystal planes, and identify planes with potential for enhanced activity for study.

The adsorption of CO on Pt(111) studied by infrared reflection-adsorption spectroscopy

The adsorption of CO on Pt(111) studied by infrared reflection-adsorption spectroscopy